Resiniferatoxin

Local Resiniferatoxin Induces Long-Lasting Analgesia in a Rat Model of Full Thickness Thermal Injury.[Pubmed:27794548]

Pain Med. 2017 Dec 1;18(12):2453-2465.

Objective: Opioid-based analgesics are a major component of the lengthy pain management of burn patients, including military service members, but are problematic due to central nervous system-mediated side effects. Peripheral analgesia via targeted ablation of nociceptive nerve endings that express the transient receptor potential vanilloid channel 1 (TRPV1) may provide an improved approach. We hypothesized that local injection of the TRPV1 agonist Resiniferatoxin (RTX) would produce long-lasting analgesia in a rat model of pain associated with burn injury. Methods: Baseline sensitivities to thermal and mechanical stimuli were measured in male and female Sprague-Dawley rats. Under anesthesia, a 100 degrees C metal probe was placed on the right hind paw for 30 seconds, and sensitivity was reassessed 72 hours following injury. Rats received RTX (0.25 mug/100 muL; ipl) into the injured hind paw, and sensitivity was reassessed across three weeks. Tissues were collected from a separate group of rats at 24 hours and/or one week post-RTX for pathological analyses of the injured hind paw, dorsal spinal cord c-Fos, and primary afferent neuropeptide immunoreactivity. Results: Local RTX reversed burn pain behaviors within 24 hours, which lasted through recovery at three weeks. At one week following RTX, decreased c-Fos and primary afferent neuropeptide immunoreactivities were observed in the dorsal horn, while plantar burn pathology was unaltered. Conclusions: These results indicate that local RTX induces long-lasting analgesia in a rat model of pain associated with burn. While opioids are undesirable in trauma patients due to side effects, RTX may provide valuable long-term, nonopioid analgesia for burn patients.

Direct influence of systemic desensitization by resiniferatoxin on the activities of Adelta- and C-fibers in the rat primary bladder mechanosensitive afferent nerves.[Pubmed:27509829]

Int J Urol. 2016 Nov;23(11):952-956.

OBJECTIVES: To examine whether systemic Resiniferatoxin treatment can desensitize the single-unit afferent activities of Adelta- and C-fibers in rat primary bladder mechanosensitive afferent nerves. METHODS: Female Sprague-Dawley rats were divided into two groups (Resiniferatoxin-treated: n = 9, vehicle-treated: n = 9). Resiniferatoxin (0.3 mg/kg) or its vehicle (10% ethanol) was injected subcutaneously after the first eye-wipe behavior test with capsaicin. The second eye-wipe behavior test was carried out 48 h after the injection under urethane (1.2 g/kg intraperitoneally) anesthesia. The bladder mechanosensitive single-unit afferent activities originating from the left L6 dorsal roots were identified by electrical stimulation of the left pelvic nerve and bladder distension, and classified by conduction velocity as either Adelta-fibers or C-fibers. The single-unit afferent activity measurements were carried out with constant bladder filling with saline (0.1 mL/min, <30 cmH2 O). RESULTS: After Resiniferatoxin treatment, but not vehicle treatment, eye-wipe behavior with capsaicin was completely abolished. A total of 104 single afferent fibers were isolated (Adelta-fibers: n = 41, C-fibers: n = 63). Single-unit afferent activities of both Adelta-fibers and C-fibers in response to bladder filling in the Resiniferatoxin-treated group were significantly lower than those in the vehicle-treated group. These differences were observed throughout the filling phase in Adelta-fibers and during the initial half of the filling phase in C-fibers. CONCLUSIONS: The present results show the existence of Resiniferatoxin desensitization-resistant mechanosensitive Adelta- and C-fibers in rat primary bladder afferent nerves, suggesting that systemic Resiniferatoxin treatment might induce non-specific partial desensitization or denervation of mechanosensitive capsaicin-insensitive Adelta- and C-fibers of the rat bladder.

Distinct structure-activity relations for stimulation of 45Ca uptake and for high affinity binding in cultured rat dorsal root ganglion neurons and dorsal root ganglion membranes.[Pubmed:8717353]

Brain Res Mol Brain Res. 1996 Jan;35(1-2):173-82.

The [3H]Resiniferatoxin (RTX) binding assay using membrane preparations has been used to identify and characterize the vanilloid receptors in the central and peripheral nervous system of different species. In the present study, using cultured adult rat dorsal root ganglion neurons either in suspension or attached to the tissue culture plates, we developed an assay to measure specific [3H]RTX binding by the intact cells. We were able to characterize the vanilloid binding characteristics of the neurons and compared those to the properties of vanilloid binding sites present in rat dorsal root ganglia membrane preparations. We found that [3H]RTX bound with similar affinity and positive cooperativity to attached neurons (cultured for 5 days before being assayed), neurons in suspension (using a filtration assay) and dorsal root ganglion membrane preparations. Dissociation constants obtained in the three assays were 47.6 +/- 3.5 pM, 38.4 +/- 3.1 pM and 42.6 +/- 3.1 pM, respectively. The cooperativity indexes determined by fitting the data to the Hill equation were 1.73 +/- 0.11, 1.78 +/- 0.12 and 1.78 +/- 0.09, respectively. The maximal binding capacity was 0.218 +/- 0.026 fmol/10(3) cells and 0.196 +/- 0.021 fmol/10(3) cells in the case of the attached cells and cells in suspension, respectively. Nonradioactive RTX, capsaicin, capsazepine and resiniferonol 20-homovanillylamide fully displaced specifically bound [3H]RTX from cells in suspension with Ki and Hill coefficient values of 42.5 +/- 5.3 pM, 2.06 +/- 0.16 microM, 3.16 +/- 0.21 microM and 32.4 +/- 4.1 nM and 1.79 +/- 0.17, 1.68 +/- 0.06, 1.72 +/- 0.11 and 1.81 +/- 0.12, respectively. Structure-activity analysis of different vanilloid derivatives revealed that the various compounds have distinct potencies for receptor binding and inducing 45Ca uptake in rat dorsal root ganglion neurons. Affinities for receptor binding and stimulation of 45Ca uptake of RTX, resiniferonol 20-homovanillylamide, RTX-thiourea, tinyatoxin, phorbol 12,13-dibenzoate 20-homovanillylamide and capsaicin were 38.5 +/- 2.9 pM, 25.7 +/- 3.0 nM, 68.5 +/- 3.8 nM, 173 +/- 25 pM, 7.98 +/- 0.83 microM and 4.93 +/- 0.35 microM as compared to 0.94 +/- 0.12 nM, 26.5 +/- 3.5 nM, 149 +/- 30 nM, 1.46 +/- 0.25 nM, 1.41 +/- 0.48 microM and 340 +/- 57 nM. Computer fitting of the data yielded Hill coefficient values indicating positive cooperativity of receptor binding; however, stimulation of 45Ca uptake appeared to follow a non-cooperative mechanism of action. The competitive capsaicin antagonist capsazepine inhibited specific binding of [3H]RTX by rat dorsal root ganglion membrane preparations with Ki and Hill coefficient values of 3.89 +/- 0.38 microM and 1.74 +/- 0.11. On the other hand it inhibited the induction of 45Ca uptake into the cells induced by capsaicin and RTX in a non-cooperative fashion with Ki values of 271 +/- 29 nM and 325 +/- 47 nM. Our results show that the membrane binding assay relates to the reality of receptor function in the intact, cultured neurons, both in terms of affinity and positive cooperativity. However the different vanilloid derivatives displayed markedly distinct structure-activity relations for high affinity receptor binding and stimulation of 45Ca uptake into rat dorsal root ganglion neurons. Among various explanations for this discrepancy, we favor the possibility that the two assays detect distinct classes of the vanilloid (capsaicin) receptor present in primary sensory neurons.

Resiniferatoxin: an ultrapotent selective modulator of capsaicin-sensitive primary afferent neurons.[Pubmed:2243359]

J Pharmacol Exp Ther. 1990 Nov;255(2):923-8.

Resiniferatoxin (RTX) has been shown to function as an ultrapotent analog of capsaicin. It is reported here that RTX, like capsaicin, acts selectively on primary sensory neurons in rats to produce ultrastructural alterations and calcitonin gene-related peptide depletion. To evaluate RTX actions on capsaicin-sensitive nociceptors of vagal origin in the lung, the activation and desensitization of the pulmonary chemoreflex in both rats and cats were examined. In rats, RTX (2 ng-5 micrograms/kg i.v.) failed to elicit the full reflex triad (apnea, systemic hypotension and bradycardia); RTX did, however, desensitize the pulmonary chemosensitive receptors to capsaicin and phenyldiguanide. This effect is not achievable upon acute capsaicin treatment. RTX pretreatment (300 micrograms/kg s.c.) of rats also abolished the neurogenic edema formation, another response mediated via capsaicin-sensitive vagal fibers, in rat trachea to challenge either by capsaicin or ether. The effect of electrical stimulation of vagal nerve was not impaired after RTX injection, indicating that RTX desensitized only the capsaicin-sensitive pathway whereas the parasympathetic pathway remained unaltered. In cats, unlike in the rat, the full pulmonary chemoreflex occurred in response to 0.1 micrograms/kg RTX. It is concluded that RTX is a selective probe for capsaicin-sensitive neural pathways but the spectrum of action of the two compounds is not identical. The failure of RTX to provoke the pulmonary chemoreflex in the rat, which is the main limiting factor in the use of capsaicin, suggests a further advantage to the use of RTX for probing capsaicin-sensitive neural pathways.

Cellular mechanism of action of resiniferatoxin: a potent sensory neuron excitotoxin.[Pubmed:2169951]

Brain Res. 1990 Jun 18;520(1-2):131-40.

The mechanism of activation of sensory neurons by the potent irritant Resiniferatoxin (RTX) was compared with that of the pungent compound, capsaicin. RTX and capsaicin evoked an inward, depolarising current associated with an increase in membrane conductance in a subpopulation of dissociated cultured neurons from rat dorsal root ganglia. RTX also evoked an uptake of 45Ca into and an efflux of [14C]guanidinium and of 86Rb from these cells but was at least 100-fold more potent than capsaicin. The levels of cGMP, but not cAMP were elevated by RTX. Prolonged exposure to RTX damaged DRG neurons by a predominantly osmotic process. RTX-sensitive cells were identified by a cobalt-staining method; neurofilament-containing DRG neurons were RTX-insensitive as were all sympathetic neurons and non-neuronal cells. Cultured DRG neurons from chick embryos were also unaffected by RTX. In a neonatal rat spinal cord-tail preparation in vitro, RTX activated capsaicin-sensitive peripheral nociceptive fibres and caused a subsequent spinal cord depolarization measured in the ventral spinal roots. Neither prolonged exposure to a phorbol ester, to desensitize/down-regulate protein kinase C, nor inhibition of protein kinase C by staurosporine affected responses produced by RTX or capsaicin. The effects of capsaicin were abolished when preparations were exposed to desensitizing concentrations of RTX. RTX therefore acts as a highly potent capsaicin analogue to activate a subpopulation of rat sensory neurons.